1. Field of the Invention
The present invention pertains to a medical ventilator with improved spontaneous triggering and cycling and to an improved method of triggering and cycling such a ventilator. In particular, the present invention pertains to a ventilator with one or more of the following features: (1) sensitivity to a patient initiated trigger that increases as the expiratory phase of the breathing cycle progresses, (2) adaptive adjustment of cycling criteria to optimize the cycling operation, and (3) triggering, cycling, or both based on patient effort, which is determined from cross-correlating multiple patient parameters.
2. Description of the Related Art
It is known to utilize a conventional ventilator or pressure support device to deliver a fluid, such as oxygen, air or other oxygen or breathing gas mixture, to an airway of patient to augment or substitute the patient""s own ventilatory effort. It is further known to operate a conventional ventilator in a variety of modes to control the four basic operations of a ventilator, which are: 1) the trigger point, which is the transition from the expiratory to the inspiratory phase of the ventilatory cycle; 2) the inspiratory phase where the ventilator delivers the flow of breathing gas; 3) the cycle point, which is the transition from the inspiratory phase to the expiratory phase, and 4) the expiratory phase. There are four primary variables or parameters that are typically monitored and used to control how a ventilator performs one or more of these four operations. These variables are the volume, pressure, flow of fluid to or from the patient, and time.
In a typical life support situation, where there is substantially no spontaneous respiratory effort by the patient, a controlled mode of ventilation is provided, where the ventilator assumes full responsibility for ventilating the patient. In this mode of ventilation, the trigger and cycle point of the ventilator are determined based on time. In other situations, where the patient exhibits some degree of spontaneous respiratory effort, an assist mode or a support mode of ventilation is typically provided. Both of these modes of ventilation cause the ventilator to augment or assist in the patient""s own respiratory efforts. In the assist mode, the determination of the ventilator trigger point is based on the action of the patient and the determination of the cycle point is determined based on time. In the support mode, both the trigger and the cycle points are patient based and not based on time. It is also known to use a combination of these two modes, referred to as an assist/control mode of ventilation. In this mode of ventilation, the ventilator triggers an inspiratory flow only if the patient fails to initiate a respiratory effort for a period of time. Thus, the trigger point is based on either a patient action or on time, if there is no patient action within a certain period of time.
In the assist, support, and assist/control modes of ventilation, it is important that the operation of the ventilator is synchronized with the patient""s spontaneous respiratory effort, so that the ventilator triggers the inspiratory flow of breathing gas at or near the time the patient begins his or her inspiratory effort, and cycles to the expiratory phase of the breathing pattern at an appropriate time, preferably when the patient begins his or her expiratory phase of the breathing cycle. Conventional ventilators operating in an assist, support, or assist/control mode of ventilation typically monitor only one patient parameter, such as the pressure, flow, or volume, and use this single monitored parameter as a variable in determining when to spontaneously trigger the delivery of the inspiratory flow. Typically, the monitored parameter is compared to a threshold, and if the threshold is exceeded, the transition from expiration to inspiration (trigger) or from inspiration to expiration (cycle) is initiated. In other pressure support devices, the current value of the monitored parameter is compared to a previous value of the same parameter, so that the ventilator triggers or cycles based on the result of this comparison. U.S. Pat. No. 5,632,269 to Zdrojkowski et al. teaches this technique referred to as xe2x80x9cshape triggering.xe2x80x9d
This one-dimensional, i.e., one parameter, comparison of either pressure, flow, or volume to a trigger threshold is disadvantageous in that it is susceptible to random fluctuations in the monitored parameter, which may result in false triggers or cycles. In which case, an operator must intervene to reduce the trigger and/or cycle thresholds or ventilator sensitivity. However, reducing the ventilator""s sensitivity can result in a greater amount of patient effort being needed before a spontaneous patient inspiration or expiration is detected, which is also disadvantageous, because a patient on a ventilator often has a weakened respiratory system to begin with.
Accordingly, it is an object of the present invention to provide a medical ventilator system that overcomes the shortcomings of conventional ventilators with improved triggering and/or cycling capability. This object is achieved according to one embodiment of the present invention by providing a ventilator system that includes a gas flow generator adapted to provide a flow of breathing gas, a gas flow controller that controls the flow of breathing gas delivered to the patient responsive to a control signal, a patient circuit adapted to communicate the flow of breathing with an airway of the patient, a flow sensor adapted to measure the flow of breathing gas in the patient circuit and to output a first flow signal indicative thereof, a pressure sensor adapted to measure a pressure of the flow of breathing gas in the patient circuit and to output a first pressure signal indicative thereof, and an exhaust assembly adapted to communicate gas from within the patient circuit to ambient atmosphere. The ventilator system also includes a controller that receives the first flow signal and the first pressure signal and outputs the control signal that controls the flow of breathing gas delivered to the patient circuit by the pressure generating system and, hence, the flow of breathing gas at a patient""s airway. In one embodiment, the controller detects the onset of the inspiratory phase of a patient""s breathing cycle for triggering the inspiratory flow of breathing gas based on such a patient""s inspiratory effort, which is determined based on both the first flow signal and the first pressure signal.
According to a further embodiment of the present invention, the controller arms or makes available for activation a plurality of triggering mechanisms over an expiratory phase of a breathing cycle to increase the ventilator system sensitivity to a patient initiated trigger as the expiratory phase of the breathing cycle progresses.
In a still further embodiment, the controller detects the onset of the expiratory phase for cycling the ventilator based on such a patient""s expiratory effort, which is determined based on both the first flow signal and the first pressure signal. This cycling feature of the present invention can be done alone or in combination with the triggering feature noted above.
In yet another embodiment of the present invention, the controller dynamically adjusts the cycling threshold criteria on a breath by breath basis so that the ventilator cycles more closely in synchronization with the patient""s expiratory effort. In this embodiment, the ventilator system monitors the patient pressure Ppatient and, more particularly, its rate of change at the end of the inspiratory phase, as well as changes in the patient flow Qpatient at the beginning portion of the expiratory phase to determine if the ventilator cycling for that breath occurred before or after the patient began exhalation, and dynamically adjusts the cycling threshold criteria in the next breath to account for the cycling synchronization error in the previous breath.
It is yet another object of the present invention to provide a method of triggering or cycling a medical ventilator that does not suffer from the disadvantages associated with conventional triggering and cycling techniques. This object is achieved by providing a method that includes: (1) generating a flow of breathing gas, (2) providing the flow of breathing gas to a patient via a patient circuit, (3) controlling the flow of breathing gas delivered to a patient responsive to a control signal, (4) measuring the flow of breathing in the patient circuit and outputting a first flow signal indicative thereof, (5) measuring a pressure of the flow of breathing gas in the patient circuit and outputting a first pressure signal indicative thereof, (6) communicating gas from within the patient circuit to ambient atmosphere, (7) detecting the onset of the inspiratory phase of a patient""s breathing cycle for triggering an inspiratory flow of breathing gas based on the patient""s inspiratory effort, which is determined based on both the first flow signal and the first pressure signal, and (8) detecting the onset of the expiratory phase of a patient""s breathing cycle for cycling purpose based on the patient""s expiratory effort, which is also determined based on both the first flow signal and the first pressure signal. It should be noted that triggering and cycling can be done independently or they can both be done during the appropriate stages of the breathing cycle.
According to a further embodiment of the present invention, a plurality of triggering mechanisms are made active during different stages of the expiratory phase of the pateint""s breathing cycle to increase the ventilator system sensitivity to a patient initiated trigger as the expiratory phase of the breathing cycle progresses.
In yet another embodiment, the present invention provides a medical ventilator system that cycles from providing an inspiratory flow of breathing gas to allowing an expiratory flow by comparing the patient flow and a cycle threshold criteria. The system further dynamically adjusts the cycling threshold criteria on a breath by breath basis based on changes in patient pressure Ppatient at the end portion of the inspiratory phase and based on changes in the patient flow Qpatient at the beginning portion of the expiratory phase, which are indicative of whether the ventilator cycling for that breath occurred before or after the patient began exhalation. In this embodiment, the cycling threshold criteria are dynamically adjusted in the next breath to account for the cycling synchronization error in the previous breath.
These and other objects, features and characteristics of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.